Transposition for electrical conductors



Filed Dec.

United States Patent 3 283 280 TRANSPOSITION FOR ELECTRICAL ONDiiClfflR Heinz G. Fischer, Muncie, ind, assignor to Westinghouse Electric Corporation, Pittsburgh, Pa, a corporation of Pennsylvania Filed Dec. 22, 1964, Ser. No. 420,331 5 Claims. (Cl. 336187) This invention relates in general to windings for electrical inductive apparatus, such as transformers, and more particularly to an arrangement of conductors in the windings of electrical inductive apparatus.

Eddy-current losses in a conductor of a transformer winding are proportional to the square of the dimension of the conductor at right angles to the leakage flux. Eddycurrent losses may thus be reduced by subdividing the required conductor area into two or more parallel connected conductive elements or strands, which are insulated from each other. To prevent an off-setting increase in losses due to circulating currents between the parallel connected strands, the relative position of the strands are transposed with respect to the leakage flux, in an attempt to obtain the same net flux linkages for each strand. If the parallel loops are long, the impedance of the loops aids in reducing circulating currents, but it is still important to obtain a highly efiicient transposition of the conductive strands.

The type of transposition utilized on tapped coils is especially important. Each tap is connected to all of the strands which make up the conductor turn, and the loops that circulating currents have to follow are thus considerably shortened. Thus, the impedance of the loops in tapped coils may be very low, and the reduction of circulating currents is almost wholly dependent upon the transposition of the conductive strands. Further, in tapped coils it is important that the conductive strands be transposed between each tap, and it is important that the transposition arrangement used obtains substantially the same net flux linkages for each strand.

Since only a portion of a coil is actually available for making transpositions, the transposition problem is compounded, as the transposition utilized must not only be highly efficient, but it must be accomplished in the shortest possible space.

Accordingly, it is an object of this invention to provide a new and improved transposing arrangement for a plurality of conductive strands.

Another object of this invention is to provide a new and improved transposing arrangement for a plurality of conductive strands that subjects each strand to substantially the same net flux linkages.

A further object of the invention is to provide a new and improved transposing arrangement for a plurality of conductive strands that is equally suitable for use with tapped or untapped coils.

Still another object of the invention is to substantially reduce the space required for performing the transposition of a plurality of conductive strands.

Another object of the invention is to perform a highly eflicient transposition of a plurality of conductive strands in a minimum of space.

Briefly, the present invention accomplishes the above cited objects by a transposing arrangement which performs both the standard transposition and the complete transposition in the same space required for the complete transposition alone, thus reducing the space required to perform both the standard and complete transpositions by substantially fifty percent. The present invention is particularly applicable to a double section coil, with each section constructed of a main, two stranded conductor, having a predetermined number of layers of main conductor per turn, or to a coil in which each layer of each in the original coil section.

turn has four individual conductors. To perform a transposition in a double section coil in which each strand of each coil section is subjected to the same net flux linkage as all of the other strands, and thus substantially eliminate losses due to circulating currents, the two strands in each main conductor must be transposed with respect to one another, commonly called the standard transposition, and each main conductor must be transposed to the layer position in the other coil section which is adjacent to the layer position in its original coil section that is symmetrically opposite to its actual location This latter transposition is commonly called a complete transposition. In order to form the complete transposition, each main conductor is bent to move from one coil section to the other, and then is bent to proceed to a position in the new coil section which is adjacent to the position in the original coil section that is symmetrically opposite to its actual position in the original coil section.

In order to perform the standard transposition in substantially the same length or space required for the complete transposition, all of the main conductors in one coil section are bent in the same direction, in the plane of that coil section, and each proceeds until it occupies the outside layer of the turn, at which point it is bent to proceed to the adjoining coil section. Each main conductor is then bent to proceed to its new layer position, adjacent to the layer of its original coil section that is symmetrically opposite to its actual layer position in its original coil section. The main conductors in the other coil section are all bent in the same direction, opposite to the direction of the adjacent bends in the other coil section, and each proceeds until it occupies the opposite outside layer, at which time it is bent to proceed to the adjoining coil section. Each main conductor is then bent to proceed to its new layer position, adjacent to the layer of its original coil section that is symmetrically opposite to its actual layer position in its original coil section. This arrangement for performing the complete transposition allows room for each main conductor to be twisted degrees about its longitudinal axis and thus perform the standard transposition, or the transposing of the two strands of each main conductor with respect to one another. This twisting of each main conductor is performed while the main conductors are performing the complete transposition, thus reducing by substantially fifty percent the space normally required to produce both the standard and complete transpositions.

If each coil turn is comprised of a plurality of layers of four single stranded individual conductors, the transposition is performed in the same manner, considering that two adjacent individual conductors are a main conductor and are a part of one of two adjacent coil sections.

Further objects and advantages of the invention will become apparent as the following description proceeds and features of novelty which characterize the invention will be pointed out in particularity in the claims annexed to and forming a part of this specification.

For a better understanding of the invention, reference may be had to the accompanying drawings, in which:

FIGURE 1 is a front elevation view, partially in section, of a transformer illustrating a pancake type coil and its associated magnetic core;

FIG. 2 is a cross'sectional, fragmentary view of a turn of a double section coil;

FIG. 3 is an elevational view of a typical turn of a double section coil, utilizing the transposition arrangement taught by this invention;

FIG. 4 is a plan view of the turn shown in FIG. 3;

FIG. 5 is a cross-sectional view of the turn shown in FIG. 3, taken at V-V;

FIG. 6 is a cross-sectional view of the turn shown in FIG. 3, taken at VIVI; and

FIG. 7 is a pictorial view of a two stranded conductor twisted about its longitudinal axis to form a standard transposition.

Referring now to the drawings, and FIG. 1 in particular, there is illustrated a transformer core and coil assembly 30 of the shell-form type, in which a plurality of flat disc or pancake type coils 32 are stacked with their windows 31 aligned and disposed in inductive relation with magnetic core sections 34 and 36. As illustrated in FIG. 1 each pancake coil includes a plurality of turns 33.

Since the magnetic core sections 34 and 36 occupy the side portions of the pancake coil 32, and the winding and tap leads occupy the top portion of the pancake coil 32, the transpositions must be made in the space indicated in FIG. 1, which is roughly between the points located where two lines disposed 45 from the vertical, starting at the two lower corners of the pancake coil 32, would intersect the coil. If the coil is untapped it is usually necessary to transpose the conductor strands only once, at a point substantially midway between their points of common connection, as nearly equal and opposite voltages will be induced in the two halves, substantially eliminating circulating currents.

If the coil is tapped, the transposing arrangement should be used between each set of taps, requiring that a plurality of transpositions be utilized. The exact number of transpositions will depend, of course, upon the number of taps. The taps reduce the length of the loops the circulating currents must follow, making it most important that the transposition arrangement subject each strand to the same net flux linkages as all the other strands, to substantially eliminate losses due to circulating currents. The amount of space available for performing the transposition on tapped coils is the same space as for untapped coils, as shown in FIG. 1. Thus, the requirements for the transposition arrangement are that it be as efficient as possible in eliminating circulating currents, and it must be accomplished in a minimum of space in order to be utilized on tapped coils, as well as on untapped coils.

Although FIG. 1 illustrates a transformer of the shellform type, the invention is not to be so limited, as it is equally applicable to transformers of the core-form type.

The present invention is particularly applicable to double section coils, in which each section includes a plurality of turns comprising a plurality of layers of multiple stranded main conductor. However, it will be obvious that the invention is equally applicable to coils in which the turns include a plurality of layers of single stranded main conductors having an even number of four or more main conductors per layer so that the coil may be divided into two equal sections. For purposes of simplicity, the invention will be described with double section coils, with each section having turns comprising a plurality of layers of double stranded main conductors, as it will be obvious from the description how the invention would be applied to a plurality of single stranded conductors.

FIG. 2 is a fragmentary view, in section, illustrating a portion of a turn of a typical double section coil. Roman numerals I and II illustrate the two coil sections, and letters A, B, C and D illustrate four of the layers of the turn, which may have as many layers as required in a particular application. Each layer of each coil section is formed from a main conductor 38 which has two conducting strands 40 and 42. Each conducting strand 40 is separately taped or wrapped with insulation 44 and 46, respectively. The insulated strands 40 and 42 are then taped or wrapped together with insulation 48, to form the main conductor 38. Each pancake type coil is formed of a plurality of turns, with each turn comprising as many layers as dictated by the particular application. The insulated conductors of each turn are connected together at the start and finish of each coil, and each coil of a winding is connected in a series, parallel,

or series-parallel relation with the other coils of its winding.

Since it is important to obtain a highly efficient transposition, it would be desirable to transpose the strands in each main conductor with respect to one another, called a standard transposition, and it would be desirable to transpose the main conductors of one coil section with the main conductors of the other coil section, called a complete transposition. However, to first perform the standard transposition and then the complete transposition would require a comparatively large space, which in many cases is not available.

This invention teaches how both the standard and complete transpositions may be made in substantially the same space required for the complete transposition alone, thus providing a highly efiicient transposition in substantially one-half the space required by prior art arrangements.

FIGS. 3 and 4 illustrate elevational and plan views, respectively, of a portion of a turn 50 of a double section coil, with each coil section I and II having a plurality of superposed layers of main conductor, with each layer of each coil section having two conductor strands. FIG. 3 illustrates the turn 50 as having six layers of main .conductor A, B, C, D, E and F, but it will be understood the invention is applicable to any number of layers.

In general, to perform the complete transposition, each main conductor of each coil section is bent so that the main conductors successively appear at one of the outer layers of its particular coil section, at which time it is bent to enter the adjoining coil section. After each main conductor enters the adjoining coil section, it is bent to proceed to its new layer location, which is adjacent to the layer of its original coil section that is symmetrically opposite to its actual layer location in its original coil section.

In order to easily identify the position of each of the two conductor strands which make up each main conductor of each coil section, before and after the transposition, sections have been taken through turn 50 at points V-V and V I-VI, and they appear as FIGS. 5 and 6, respectively. Each strand has been given a reference numeral so that the effect of the transposition will be readily apparent by observing FIGS. 5 and 6. Since FIG. 2 illustrates the actual construction of the conductors, FIGS. 5 and 6 illustrate the conductors and their individual strands in a functional manner.

More specifically, as shown in FIG. 3, the transposition starts at point 52 and ends at point 53 with all of the bends taking place in the transition zone between point 52 and 53. At this start 52 of the transposition, the main conductors of coil section II are all bent in the same direction, which in this instance is upward, with the bends being in the same plane as coi-l section II. The main conductors of section I, at the start 52 of the transposition, are all bent in the same direction, in the plane of coil section I, and opposite to the direction of the adjacent bends in coil section II. The upward bend of the main conductors of coil section II at point 52, causes each main conductor to successively appear at the outer layer of coil section II. when each main conductor of coil section 11 reaches the outer layer of coil section II, it is bent away from the plane of coil section II and towards coil section I. Once a main conductor from coil section II enters the plane of coil section I, it is bent to coincide with the plane of coil section I and is also bent to give it is a predetermined downward angle. Each main conductor is allowed to continue in coil section I at the predetermined downward angle until it reaches the layer position in coil section I that is adjacent to the layer in coil section II that is symmetrically opposite its actual layer location in coil section II. At this point, the main conductor is bent at point 53 to enter this layer position, and the transposition of the conductors from coil section II to coil section I is completed. For example, main conductor '56 is bent at 78 with a predetermined upward angle. At 80, main conductor 56 occupies the outside layer and is bent towards coil section I. At $2 conductor 56 is bent with a predetermined downward angle, in the plane of coil section I, until conductor 56 reaches a layer position that is adjacent the layer of coil section II that is symmetrically opposite to its actual layer position in coil section II. At this point, given the reference numeral 84, main conductor 56 is bent again to enter the new layer position, and complete the transposition.

In other words, the complete transposition produces a 180 degree rotational symmetry of the main conductors. First, assume a central axis 95 in the turn 50 shown in FIG. 5, located at the intersection of a longitudinal plane 99 which separates coil sections I and II, and a transverse plane 97 which divides the turn into two equal upper and lower sections, as viewed in FIG. 5. Then, if the turn 50 is rotated on this axis 180 degrees, the main conductors will all take the positions shown in FIG. 6. Thus, the respective locations of the main conductors before and after the complete transposition, as shown in FIGS. 5 and 6, are in 180 degrees rotational symmetry.

Another Way of describing the complete transposition is to take two successive mirror images, first using a first plane which divides the turn into two equal parts for the first mirror image, and then a second plane for the second mirror image which is perpendicular to the first plane, and which again divides the turn into two equal parts. For example, the successive mirror images could first use plane 99 and then plane 97 or the successive mirror images could first use plane 97 and then plane 99.

The main conductors of coil section I are all bent with a downward angle in the plane of coil section -I, at the start 52 of the transposition, and they successively appear at the lower outside layer of coil section I. When each main conductor of coil section I reaches the lower outside layer of coil section I, it is bent away from the plane of coil section I and toward the plane of coil section II. Once a main conductor from coil section I enters the plane of coil section II, it is bent to coincide with the .plane of coil section II and is also bent with a predetermined upward angle. Each main conductor is allowed to continue in coil section II at the predetermined upward angle until it reaches a layer position in coil section II that is in 180 degree rotational symmetry with its layer position in coil section I. At this point, the main conductors are bent, as shown in 53 to enter this new layer position, and the transposition of the conductors from coil section I to coil section II is complete. For example, conductor 68 is bent at 90 with a predetermined downward angle. At 92, main conductor 68 has reached the outside layer and is bent towards coil section II. At 94 main conductor 68 is bent with a predetermined upward angle to follow the plane of coil section II to its transposed layer position, in 180 degree rotational symmetry with its previous layer position. At 96, main conductor 68 is bent again to enter this new layer position and complete the transposition.

It will be observed that this sequence performs a complete transposition, with each main conductor of each coil section occupying a position in the opposite coil section that is in 180 degree rotational symmetry with its former position. This location of each main conductor, before and after the complete transposition, being in symmetrical relation with the two perpendicular imaginary center lines 97 and 99 which equally divide the two coil sections, subjects each main conductor to substantially the same net flux linkages, reducing the circulating currents between the main conductors to a minimum.

It is important to note that the method of performing the complete transposition described herein, wherein each main conductor proceeds to the outside layer of its associated coil section before entering the adjacent coil section, causes all of the bends from one coil section to the other to take place at the outer edges of the turn.

It will be noted that while the arrangement hereinbefore described performs a complete transposition, or a transposition of every main conductor from one coil section to a position in the adjacent coil section that is in 180 degree rotational symmetry with its previous position, that the individual conductor strands of each main conductor have not been transposed with respect to one another. The transposition of the conducting strands of each main conductor is called a standard transposition, and in addition to the complete transposition is necessary to obtain a highly efiicient transposition and reduce circulating currents to an absolute minimum.

The standard transposition of the two conducting strands of each main conductor is accomplished by twisting each main conductor 180 degrees about its longitudinal axis 101, as shown in FIG. 7. By twisting each main conductor 180 degrees about its central or longitudinal axis 101, such as main conductor 100, the relative positions of conducting strands 102 and 104 are transposed with respect to one another.

Instead of performing the standard transposition of FIG. 7 remote from the complete transposition shown in FIG. 3, which would substantially increase the space or length required for each transposition arrangement, the complete transposition arrangement hereinbefore described allows the standard transposition to be performed in the transition zone of the complete transposition. Thus, the standard transposition is performed within the same space required for the complete transposition.

The complete transposition arrangement shown in FIG. 3, ensures that each main conductor takes the same distance to perform the complete transposition as all the other main conductors. This aids in producing the same net flux linkages for each conducting strand, and also allows space for the standard transposition to be performed while the complete transposition is being performed.

More specifically, referring again to FIG. 3, standard transpositions of the strands of each main conductor may be accomplished during the complete transposition of the main conductors. Main conductors 60, 62 and 64 from coil section II are twisted about their longitudinal axes at 110, 111 and 112, while still in coil section II, to perform the standard transposition of their conducting strands.

In like manner, main conductors 70, 74 and 76 from coil section I are twisted about their longitudinal axes at 114, 116 and 118 respectively, while still in coil section I, to perform the standard transposition of their strands.

The remaining standard transpositions occur after the main conductors have already entered the adjacent coil section. Main conductors 66, 68 and 72 from coil section I are twisted about their longitudinal axes after being transposed to coil section II, as shown at 120, 122 and 124, respectively. In like manner, main conductors 56, 58 and 54, from coil section II, are twisted about their longitudinal axes after being transposed to coil section I, as shown at 126, 128 and 130, respectively. Insulating members may be disposed between the standard transpositions and the adjacent main conductors, to ensure that edge contacts will not occur that may cause an eventual insulation failure.

It will be noted that the standard transpositions 119 and 116, 112 and 114, 111 and 126, 118 and 120, 122 and 128, and 124 and 130, each appear adjacent one another in the two coil sections, and that certain of the standard transpositions are vertically aligned in each coil section, with one main conductor disposed between them. This arrangement insures a mechanical symmetry of the transposition and allows insulating members 140, which protect the edges of the main conductors during the degree twist, to be smoothly fitted and interleaved.

It is to be understood that the invention is not to be limited to the specific embodiment wherein the standard transpositions are disposed adjacent one another, however, as they may be staggered and still produce the standard transposition within the same space required for the complete transposition.

Thus, a standard and complete transposition have been completed in substantially the same space required for the complete transposition alone. For example, by comparing FIGS. and 6, it will be noted that main conductor 76 of coil section I, which occupied the upper layer before the transposition, has taken a position in the adjacent 'coil section II, after the transposition, that is in rotational symmetry with its previous position. By further observing main conductor 76 before the transposition, it will be noted that its conducting strands 1 and 2, are arranged with conducting strand 1 to the left of conducting strand 2. After the transposition, conducting strand 2 is disposed to the left of conducting strand 1.

In like manner, it will be noted that every main conductor has been transposed to the adjacent coil section, where it occupies a layer position in rotational symmetry with its previous layer position, and the individual conducting strands of each main conductor have been transposed relative to one another.

There has, therefore, been disclosed a new and improved transposition arrangement ztor a plurality of stranded main conductors, in which the main conductors have been transposed relative to one another, and the individual strands of each main conductor have been transposed relative to one another. Further, the transposing of the individual strands of each main conductor has been accomplished during the transposition of the main conductors, to reduce by substantially fifty percent the space required to perform the two types of transpositions.

Since numerous changes may be made in the above described apparatus and different embodiments of the invention may be made without departing from the spirit thereof, it is intended that all matter contained in the foregoing description or shown in the accompanying drawings shall be interpreted as illustrative, and not in a limiting sense.

I claim as my invention:

1. A transposing arrangement for electrical conductors comprising:

a plurality of main electrical conductors disposed to form two sections in side-by-side relation,

each of said sections having a plurality of superposed layers of said main electrical conductors,

each of said main electrical conductors having a plurality of conductive strands,

each of said plurality of main electrical conductors of one of said sections being bent in the plane of that section towards one of the outer layers of said sections, each of said plurality of main electrical conductors of the other section being bent in the plane of that section towards the other outer layer of said sections,

each main conductor being bent to enter the adjacent section at one of the outer layers of said sections,

each main conductor being bent in the plane of its new section to form a layer which is in 180 degree rotational symmetry with its layer position in its original section,

the first and last bends of said main conductors defining a transition zone of predetermined length,

each of said main conductors being twisted 180 degrees about its longitudinal axis in said transition zone to change the relative positions of said plurality of conducting strands.

2. A transposing arrangement for electrical conductors comprising:

a plurality of main electrical conductors disposed to form two sections in side-by-side relation,

8 each of said sections having a plurality of superposed layers of said main electrical conductors, each of said main electrical conductors having two conductive strands, each of said plurality of main electrical conductors of one of said sections being bent in the plane of that section towards one of the outer layers of said sections, each of said plurality of main electrical conductors of the other section being bent in the plane of that section towards the other outer layer of said sections, each main conductor being bent to enter the adjacent section at one of the outer layers of said sections, each main conductor being bent in the plane of its new section to form a layer which is in rotational symmetry with its layer position in its original section, the first and last bends of said main conductors defining a transition zone having a predetermined length, each of said main conductors being twisted degrees about its longitudinal axis in said transition zone to change the relative positions of the two conducting strands in each of said main electrical conductors, the same number of twists occurring in each section. 3. A transposing arrangement for electrical conductors comprising:

a plurality of main electrical conductors disposed to form two sections in side-by-side relation, each of said sections having a plurality of superposed layers of said main electrical conductors, each of said main electrical conductors having two conductive strands, each of said plurality of main electrical conductors of one of said sections being bent in the plane of that section towards one of the outer layers of said sections, each of said plurality of main electrical conductors of the other section being bent in the plane of that sec tion towards the other outer layer of said sections, each main conductor being bent to enter the adjacent section at one of the outer layers of said sections, each main conductor being bent in the plane of its new section to form a layer which is in rotational symmetry with its layer position in its original section, the first and last bends of said main conductors defining a transition zone having a predetermined length, each of said main conductors being twisted 180 degrees about its longitudinal axis in said transition zone to change the relative positions of the two conducting strands in each of said main electrical conductors, the same number of twists occurring in each section, each of the twists in one coil section being disposed opposite a twist in the adjacent coil section, said twists in each section being disposed with at least one main conductor between them. 4. A transposing arrangement for reducing circulating currents between a plurality of parallel connected electrical conductors comprising, an electrical coil, said electrical coil having a plurality of turns, with each turn including two sections, each section of each turn being formed of a plurality of layers of superposed main electrical conductors, said main electrical conductors each having a plurality of conductive strands, each of said main electrical conductors in both sections being bent in the plane of their respective sections, with the main electrical conductors of one section all being bent towards one of the outer layers of said sections and the main electrical conductors of the other section all being bent in the opposite direction towards the opposite outer layer of said sections, each main conductor being bent to enter the adjacent section at one of the outer layers of said sections, each main conductor being bent in the plane of its new section to form a layer which is in 180 degree rotational symmetry with its layer position in its original coil section, the first and last bends of said main conductors defining a transition zone of predetermined length, each of said main conductors being twisted 180 degrees about its longitudinal axis in said transition zone to change the relative positions of said plurality of conductive strands in each of said main electrical conductors.

5. A transposing arrangement for reducing circulating currents between a plurality of parallel connected electrical conductors comprising, an electrical coil, said electrical coil having a plurality of turns, with each turn including two sections, each section of each turn being formed of a plurality of layers of superposed main electrical conductors, said main electrical conductors each having a plurality of conductive strands, all of the conductive strands being connected together at the start and finish of said elecrical coil, a plurality of spaced taps each connected to all of the conductive strands of predetermined turns, each of said main electrical conductors in both sections of a portion of the turn between each set of taps being bent in the plane of their respective sections, with the main electrical conductors of one section all being bent towards one of the outer layers of said sections, and the main electrical conductors of the other section all being bent towards the opposite outer layer of said sections, each main conductor being bent to enter 10 the adjacent section at one of the outer layers of said sections, each main conductor being bent in the plane of its new section to form a layer which is in 180 degree rotational symmetry with its layer position in its original coil section, the first and last bends of said main conductors defining a transition zone of predetermined length, each of said main conductors being twisted 180 degrees about its longitudinal axis in said transition zone to change the relative positions of said plurality of conductive strands in each of said main electrical conductors.

References Cited by the Examiner UNITED STATES PATENTS 531,614 12/1884 Guilleaume 17434 2,978,530 4/1961 Braeckman 174-34 3,118,015 1/1964 Willyoung 336-187 X 3,145,358 8/1964 Sealey 336188 X LEWIS H. MYERS, Primary Examiner. LARAMIE E. ASKIN, Examiner. D. J. BADER, Assistant Examiner. 

1. A TRANSPOSING ARRANGEMENT FOR ELECTRICAL CONDUCTORS COMPRISING: A PLURALITY OF MAIN ELECTRICAL CONDUCTORS DISPOSED TO FORM TWO SECTIONS IN SIDE-BY-SIDE RELATION, EACH OF SAID MAIN HAVING A PLURALITY OF SUPERPOSED LAYERS OF SAID MAIN ELECTRICAL CONDUCTORS, EACH OF SAID MAIN ELECTRICAL CONDUCTORS HAVING A PLURALITY OF CONDUCTIVE STRANDS, EACH OF SAID PLURALITY OF MAIN ELECTRICAL CONDUCTORS OF ONE OF SAID SECTIONS BEING BENT IN THE PLANE OF THAT SECTION TOWARDS ONE OF THE OUTER LAYERS OF SAID SECTIONS, EACH OF SAID PLURALITY OF MAIN ELECTRICAL CONDUCTORS OF THE OTHER SECTION BEING BENT IN THE PLANE OF THAT SECTION TOWARDS THE OTHER OUTER LAYER OF SAID SECTIONS, EACH MAIN CONDUCTOR BEING BENT TO ENTER THE ADJACENT SECTION AT ONE OF THE OUTER LAYERS OF SAID SECTIONS, EACH MAIN CONDUCTOR BEING BENT IN THE PLANE OF ITS NEW SECTION TO FORM A LAYER WHICH IS IN 180 DEGREE ROTATIONAL SYMMETRY WITH ITS LAYER POSITION IN ITS ORIGINAL SECTION, THE FIRST AND LAST BENDS OF SAID MAIN CONDUCTORS DEFINING A TRANSITION ZONE OF PREDETERMINED LENGTH, EACH OF SAID MAIN CONDUCTORS BEING TWISTED 180 DEGREES ABOUT ITS LONGITUDINAL AXIS IN SAID TRANSITION ZONE TO CHANGE THE RELATIVE POSITION OF SAID PLURALITY OF CONDUCTING STRANDS 